Over-voltage and over-current protection mechanisms are provided in a conventional step-up current-mode converter to prevent the inductor current and the output voltage of the converter from being excessive. As shown in FIG. 1, a conventional asynchronous step-up current-mode converter 100 comprises a controller 102 having a pin EXT to output a control signal S1 to switch a transistor 108 connected between a phase node 106 and ground GND, by which, when the transistor 108 turns on, an inductor current Iin is produced and flows through the transistor 108 and an inductor 104 connected between an input voltage Vin and the phase node 106, and when the transistor 108 turns off, the inductor current Iin flows through a diode 110 to charge a capacitor Co, to thereby produce an output voltage Vout. Two resistors R1 and R2 are connected in series between the converter output Vout and ground GND to divide the output voltage Vout to produce a voltage VFB fed back to a pin FB of the controller 102 so as to determine the signal S1. To prevent the inductor current Iin from being so large to damage the transistor 108, the controller 102 uses a pin CS connected to the phase node 106 through a resistor Rcs for sensing the inductor current Iin, by which, when the inductor current Iin is greater than a threshold value, the controller 102 will turn off the transistor 108 to avoid damage to the transistor 108. On the other hand, to prevent the output voltage Vout from being so high to damage the load circuit connected to the converter output Vout, the controller 102 uses a pin OVP to sense the output voltage Vout, and when the output voltage Vout is greater than a threshold value, the controller 102 will turn off the input voltage Vin or ground the converter output Vout to avoid damage to the load circuit connected to the converter output Vout.
Thus, to achieve over-voltage and over-current protection, the conventional step-up current-mode converter 100 is required two pins CS and OVP to separately sense the inductor current Iin and the output voltage Vout, and each additional pin requires more chip area and cost for the controller 102. Particularly for a multi-channel step-up current-mode converter, for example an N-channel one as shown in FIG. 2, a controller 150 therefor is required two pins OVP_J and CS_J (J=1,2, . . . , N) for each channel to achieve over-voltage and over-current protection, thus the pin count of the controller 150 increases dramatically.
Therefore, it is desired an apparatus and method to achieve over-voltage and over-current protection by utilizing fewer pins.